Flash - CAIRN RESEARCH - #19

__Running Experiments

The power supply unit is switched on by depressing the red push-button at the top left of the front panel. The push-button illuminates to indicate that the unit is on. Press it again to switch off. Next to it is a green push-button. This is illuminated when the flash storage capacitance is fully charged to its selected voltage. A flash can be elicited manually by pressing this button. Unlike the mains switch, it has a momentary action, i.e. it immediately returns to its off state when it is released.

A flash can also be elicited by applying a pulse input to the external input BNC connector. The pulse should be positive-going, and nominally of 5V amplitude and 1msec or more in duration, but satisfactory results may also be achieved by values outside this range. The input is optically isolated, so it is important to make both the inner and the outer connections to the BNC socket in order to complete the electrical circuit. The other way of eliciting flashes is to switch the unit into "test" mode using the front panel switch. Flashes will be produced repeatedly at about three per second, and this mode is intended for setting up and aligning the optical pathway. For proper operation in test mode it is important that the capacitance and voltage values (see later) are both kept low enough for the unit to recharge fully between flashes.

Whenever a flash is produced, an output pulse appears on the second BNC socket. This pulse is 2msec in duration and approximately 5V in amplitude. It is also optically isolated. Please note that because of the optical isolation, its current-sourcing capability is somewhat more limited than a direct electrical output would be, and we recommend that it is used to drive load impedance's of 10,000 ohms or greater, otherwise the amplitude of the pulse may be reduced. If in doubt, you can monitor the output on an oscilloscope. As well as acting as a monitor, this output is also intended for use when simultaneous fluorescence measurements are being made with Cairn equipment. Our photomultiplier housing has a "gate" input, which switches off the photomultiplier tube when a signal is applied to it. By connecting this output to the housing's gate input, the photomultiplier will be switched off for the duration of the flash, preventing the fluorescence measurement electronics from being overloaded if a significant amount of flash light reaches the photomultiplier. The output can produce sufficient current to drive the gate inputs of either one or two photomultipliers.

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